diff --git a/activesupport/lib/active_support/core_ext/string/output_safety.rb b/activesupport/lib/active_support/core_ext/string/output_safety.rb
index e3fa528efb419..4089b02d4503d 100644
--- a/activesupport/lib/active_support/core_ext/string/output_safety.rb
+++ b/activesupport/lib/active_support/core_ext/string/output_safety.rb
@@ -98,19 +98,31 @@ def initialize
       end
     end
 
+    def [](*args)
+      return super if args.size < 2
+
+      if html_safe?
+        new_safe_buffer = super
+        new_safe_buffer.instance_eval { @html_safe = true }
+        new_safe_buffer
+      else
+        to_str[*args]
+      end
+    end
+
     def safe_concat(value)
-      raise SafeConcatError if dirty?
+      raise SafeConcatError unless html_safe?
       original_concat(value)
     end
 
     def initialize(*)
-      @dirty = false
+      @html_safe = true
       super
     end
 
     def initialize_copy(other)
       super
-      @dirty = other.dirty?
+      @html_safe = other.html_safe?
     end
 
     def clone_empty
@@ -120,7 +132,7 @@ def clone_empty
     end
 
     def concat(value)
-      if dirty? || value.html_safe?
+      if !html_safe? || value.html_safe?
         super(value)
       else
         super(ERB::Util.h(value))
@@ -133,7 +145,7 @@ def +(other)
     end
 
     def html_safe?
-      !dirty?
+      defined?(@html_safe) && @html_safe
     end
 
     def to_s
@@ -161,18 +173,12 @@ def #{unsafe_method}(*args, &block)       # def capitalize(*args, &block)
           end                                       # end
 
           def #{unsafe_method}!(*args)              # def capitalize!(*args)
-            @dirty = true                           #   @dirty = true
+            @html_safe = false                      #   @html_safe = false
             super                                   #   super
           end                                       # end
         EOT
       end
     end
-
-    protected
-
-    def dirty?
-      @dirty
-    end
   end
 end
 
diff --git a/activesupport/test/safe_buffer_test.rb b/activesupport/test/safe_buffer_test.rb
index e731f1c2e7cd1..25c95f2a2f365 100644
--- a/activesupport/test/safe_buffer_test.rb
+++ b/activesupport/test/safe_buffer_test.rb
@@ -89,25 +89,32 @@ def test_titleize
     assert_equal "hello&lt;&gt;", clean + @buffer
   end
 
-  test "Should concat as a normal string when dirty" do
+  test "Should concat as a normal string when safe" do
     clean = "hello".html_safe
     @buffer.gsub!('', '<>')
     assert_equal "<>hello", @buffer + clean
   end
 
-  test "Should preserve dirty? status on copy" do
+  test "Should preserve html_safe? status on copy" do
     @buffer.gsub!('', '<>')
     assert !@buffer.dup.html_safe?
   end
 
-  test "Should raise an error when safe_concat is called on dirty buffers" do
+  test "Should return safe buffer when added with another safe buffer" do
+    clean = "<script>".html_safe
+    result_buffer = @buffer + clean
+    assert result_buffer.html_safe?
+    assert_equal "<script>", result_buffer
+  end
+
+  test "Should raise an error when safe_concat is called on unsafe buffers" do
     @buffer.gsub!('', '<>')
     assert_raise ActiveSupport::SafeBuffer::SafeConcatError do
       @buffer.safe_concat "BUSTED"
     end
   end
-  
-  test "should not fail if the returned object is not a string" do
+
+  test "Should not fail if the returned object is not a string" do
     assert_kind_of NilClass, @buffer.slice("chipchop")
   end
 
@@ -119,4 +126,23 @@ def test_titleize
     assert @buffer.clone_empty.html_safe?
     assert !@buffer.gsub!('', '').clone_empty.html_safe?
   end
+
+  test "Should be safe when sliced if original value was safe" do
+    new_buffer = @buffer[0,0]
+    assert_not_nil new_buffer
+    assert new_buffer.html_safe?, "should be safe"
+  end
+
+  test "Should continue unsafe on slice" do
+    x = 'foo'.html_safe.gsub!('f', '<script>alert("lolpwnd");</script>')
+
+    # calling gsub! makes the dirty flag true
+    assert !x.html_safe?, "should not be safe"
+
+    # getting a slice of it
+    y = x[0..-1]
+
+    # should still be unsafe
+    assert !y.html_safe?, "should not be safe"
+  end
 end